1. Field of the Invention
The present invention relates generally to a system and method for encoding component video signals to composite form. More particularly, it relates to such a system and method in which encoding is accomplished with all digital circuitry to provide consistent and predictable encoding results without annoying luminance and color cross-products. Most especially, it relates to such a system and method for encoding component video signals to composite form in which there is a frequency shift between the input component signals and the output composite signals.
2. Description of the Prior Art
The state of the art of encoding video signals to composite form from component form is indicated by Clarke, "Colour Encoding and Decoding Techniques for Line-Locked Sampled PAL and NTSC Television Signals," BBC Research Department, Report BBC RD 1986/2, March, 1986. Since the beginning of composite video, the uncertainties of the encoding process and the effects of the cross-luminance and cross-color products have plagued the pursuit of picture quality and consistency.
As disclosed in the above report, when component input video signals are encoded to composite form, there is usually a change in frequency from the input component signals and the output composite signal. For example, digital D1 standard input component signals have a frequency of 13.5 MHz, and digital D2 standard composite output signals have a frequency of four times the subcarrier frequency, or 14.3 MHz for NTSC signals and 17.7 MHz for PAL signals. As a result, a sample rate conversion is required during the encoding process in order to produce consistency of sampling. Changing the sampling rate results in phase differences of the samples with respect to the input composite signals. The additional samples required in order to increase frequency between the input component signals and the output composite signals are obtained by an interpolation process. The conventional approach uses a doubled sampling rate to create two interpolated samples in each half cycle of the input component signals. Using a double sampling rate requires a doubled clock frequency for the sampling circuits. It would be desirable to be able to provide a suitable change in the sampling rate using a lower clock frequency.
In the encoding of component video signals to composite video signals, the luminance component and chrominance component are processed separately prior to their additive combination to produce the composite video signals. The luminance and chrominance components each provide information for the visible portion of the component signal and the processing adds a non-visible portion, provided during the blanking interval to control the visible portion. When the two components are combined, abrupt transitions are produced between the visible and non-visible portions of the composite video signals.